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REFERENCE

IMBS

PUBLICATIONS

^^mmiliPi' STANDARDS i TECH R.I.C.

, ^ A11 102624020 NBS TECHNICAL NOTE 1162

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Transportable 1000 pF

Standard for the NBS

Capacitance Measurement

Assurance Program

-QC100

U5753

#1162

1982
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. TheBureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

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physical and chemical and materials measurement; coordinates the system with measurement

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physical and chemical measurement throughout the Nation's scientific community, industry,

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economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardizafion activities; provides scientific and technological advisory services and assistance to Federal agencies; and

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'Headquarters and Laboratories at Gailhersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. ^Some divisions within the center are located at Boulder, CO 80303.
Transportable 1000 pF Standard for the NBS

Capacitance Measurement Assurance Program

George Free and Jerome Morrow

Electrical Measurements and Standards Division

National Bureau of Standards

Washington, DC 20234

%. V ,. V ^

U)

5^

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

Issued October 1982
National Bureau of Standards Technical Note 1162

Natl. Bur. Stand. (U.S.), Tech. Note 1162, 15 pages (Oct. 1982)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982

For Sale by the Superintendent of Documents, U.S. Government Printing Office, Washiington, D.C. 20402.

Price $2.75

(Add 25 percent for other than U.S. mailing.)
TRANSPORTABLE 1000 pF STANDARD FOR THE NBS

CAPACITANCE MEASUREMENT ASSURANCE PROGRAM

George Free and Jerome Morrow

Electrical Measurements and Standards Division

National Bureau of Standards

Washington, DC 20234

Abstract

A capacitance transport standard has been constructed for

use in the National Bureau of Standards Measurement Assurance

Program, The transport standard was designed so that variations

in ambient temperature and mechanical shock would have a minimal

effect on the value of the internal reference capacitors. A

significant improvement in stability of 1000 pF capacitors during

shipment and in the laboratory has been achieved through this

desi gn

Key words: calibration; measurement assurance; measurement assurance programs;

reference standards; standard capacitors; standard qualification; transfer

standards

Int roduct i on The National Bureau of Standards Measurement Assurance

Program (MAP) in electrical units has been in existence for

several years. In this program, NBS transport standards are

shipped to customer laboratories and calibration of the

customer's reference standards is done in place. The accuracy of

the calibration procedure depends on the degree of knowledge of

the value of the NBS transport standard while it is at the

customer's laboratory; this is optimized by analysis of measurements made at NBS before and after the period of time the

standard is at that laboratory. If the value of the transfer

standard is known accurately, the advantages to the laboratory

are threefold: reduction of down-time due to the absence of

reference standards, greater confidence in the values of

laboratory standards, and documentable verification of the

customer's measurement process through the test procedure.

For the capacitance MAP, it was necessary to develop a transport standard which would behave predictably over an extended period during which the internal capacitors would be

subjected to both mechanical and thermal shock. Due to customer

needs, the transport standard would have to be at the 1000 pF

level (the most common reference standard in most laboratories).

At the 1000 pF level, the most stable, commerciallyavailable standards are nitrogen dielectric capacitors. The

values of these capacitors vary due to temperature (with

coefficients of about 2 ppm/°C), temperature hysteresis, and

mechanical stress. The reduction of instability from these

sources were planned through design of the transport box and

minor modifications to commercially available capacitors.

The goals for the construction of the transport package were

as f ol lows:
The package should provide a transportable temperature

controlled environment with a maximum departure from

nominal temperature of 0.01 °C under normal laboratory

conditions.

The package should provide for a minimum amount of

change due to handling.

The package should be of simple design and construction

so that it may be duplicated by laboratories interested

in increasing the stability of their standards.

Constructi on The most important limitation in the performance of the

transport sta^idard is the quality of the capacitor used. Tests

were devised by which the mechanical stability of capacitors

could be systematically evaluated (1) and the results were used

to select the four "best" capacitors available for inclusion in

the standard. The method of testing and the results are

discussed in the Appendix. These tests were also used to

evaluate 10 pF and 100 pF standards.

Before placing the capacitors in the transport box, their

outer enclosures were removed. Since the stability of the

trimmer in these capacitors is unknown and difficult to analyze,

it was also removed and electrical connection to the inner

enclosure was made through a special head. The head was designed

to eliminate stray capacitance between the leads coming out of

the capacitor and to guarantee good electrical contact between

the cable shield and the enclosure, thus making the capacitor more

closely meet the requirements for three-terminal devices with a negligible amount of stray capacitance.

The capacitors were placed in an aluminum box (see Figure 1)

25.4 cm (10 inches) on a side, with sufficient soft-sponge

packing material surrounding them so that they were effectively

fixed in position with no rigid connections. The capacitors were

insulated from each other by plastic sheets, thereby reducing the

possibility of electrical ground loops during measurements.

Heaters, consisting of many turns of magnet wire, were positioned

to cover the total surface of each side of the box and were

attached to the box with adhesive tape (adhesive agent on both

sides). Additional tape (of the type used to seal heating air

ducts) was tightly wrapped on top of the heater wires to assure

good thermal contact with the box and prevent motion of the wire.

An outer box was made out of plastic and wood, its dimensions

allowing for at least 7.6 cm (3 inches) of insulation between it

and the inner box. The aluminum box was then foamed in place

with rigid polyurethane. Coaxial leads from the inner box to the

outside were placed so. as to assure good thermal contact with the

inside aluminum box, reducing temperature variations transmitted

through the leads. The coaxial leads are RG316/U 50 ohm cable.

The small c ross -sect i on of metal in the inner conductor and

shielding reduces the heat path to the outside.
The temperature control circuit for the standard (see Figure

2) consists of a thermistor bridge and a very simple operational

amplifier circuit. The temperature-sensitive components of the

control circuit (resistors and amplifier) are placed inside the

aluminum box in a section separated from the capacitors by an

aluminum plate. The less sensitive components, such as the power

supply and the power transistor for the heating coils, are in a small control box separate from the transport standard. The

control thermistor is wrapped in aluminum foil and placed under

the adhesive tape between the heater wires and the aluminum

end osure.

A temperature measuring device consisting of a thermistor

bridge with variable decade resistors in one arm was constructed.

The thermistor was placed on top of and taped to one of the

capacitors.

The control circuit was designed to operate from 110 VAC or

a 12 VDC primary power source. When operated

source, the circuit needs an additional jfl5

provide current for the operational amplifier.

from a DC power

VDC battery to

Performance

As the standard was too large to fit in available

environmental chambers, the temperature-variation performance

could be studied only with respect to changes in the laboratory

environment. The temperature variation tests were carried out

over a relatively narrow range of temperatures.

For a temperature range of 3.9 °C in the laboratory, i.e.,

from 19.9 °C to 23.8 °C, the measured variation in box

temperature was .011°C while, for a temperature variation of

1.3 °C from 19.9 °C to 21.2 °C, the measured variation was

0.005°C. These two tests were carried out in different

laboratory environments.

Tests correlating change in capacitance with temperature

variations were first carried out using the measurement system

described in (1). The standards were intercompared with 100 pF

fused silica dielectric capacitors which were in a temperature

controlled air bath. The small apparent changes in value of the

transport standard could not definitely be attributed to the

transport standard alone due to possibilities of error in the

rest of the system.

The reported measurements of temperature dependence were

obtained using the measurement system described in (2) and 100 pF

quartz dielectric capacitors in an oil bath as the reference

standard. The results of the stability test are shown in Table

1. The period during which measurements were made was one month

(31 days). There were 17 i ntercompari sons and the time-of-day of

measurement was varied in order that the full range of laboratory

temperatures, i.e., 19.9 to 21.1 °C, was included. The
CONTROL

CIRCUIT

CAPACITORS

IN SOFT

FOAM

HEATER

WIRE

THERMISTOR

H^h^-^^-^^

COAXIAL LEADS

Schematic diagram of the aluminum box for housing the 1000 pF

transportable standard.

A/W^

THERMISTOR

HEATER

2 VDC

FIG. 2 Schematic diagram of heater control circuit.

4
resolution of the measuring system is j^O.OOl ppm. The range of

capacitance change, C(MAX)-C(MIN), for each standard is given in

the first row of the table.

Table 2 is based upon the same data as Table 1 but the

measured values are represented by the equation y = a + 6t +yAT

using a least squares fit. In this equation y is the departure

from the nominal capacitance value (in ppm) after a time t (in

days), and for the temperature increment AT (degrees celsius)

which is the departure of the box temperature from the nominal

control temperature, or AT=TgQ^-30.00 °C. The paramters «, 3,

and Y are, respectively, the residual capacitance offset (ppm),

the daily drift rate (ppm/day), and the temperature coefficient

(ppm/°C).

Comparisons of the residual standard deviation a (RES) with

the standard deviation of an individual measurement cr(Ind. Meas.)

of Table 1 indicates that this equation fits the performance of

the capacitors quite well. The range of temperature variations

of the standards during this test was 0.005 °C. The similarity

of Q (MEAN) in Table 1 and a(RES) in Table 2 indicates that over

a short period of time (one month) the values of the standards

could be calculated either from the mean of a set of measurements

or by using a linear fit.

The methods used to test mechanical stability of individual

capacitors could not be applid to the entire transport standard

because of its size and weight. Mechanical stability was

determined through actual shipment of the box to laboratories.

In this evaluation the capacitors were shipped five times to

various parts of the United States. All shipping was by air

freight with hand carried delivery service to and from the airports. The transport standard was shipped under battery power

for temperature control. No care was taken to ensure that the

shipping container received special handling by the airlines.

With data from shock recorders on similar boxes it was surmised

that the container might well experience forces exceeding 45g

during transport. Table 3 indicates the effects of travel on the

standards . The numbers given diVQ absolute values, the

change being disregarded. Actual changes were both

negative, thus the change in the box mean

the changes in individual capacitors,

changes due to one complete round trip.

1 ated as

and C ^ is

indicates

Co-Cy^ = change, where Cg is the

he value after shipment. The

the changes CD-Cn» where C

standard at the start

capacitor nine months

changes occurred in all

^

B "^ A*

of testing and

later. As a capacitors on

A

sign of the

positive and

i s general ly less than

The columns indicate

Each change was calcuvalue before shipment

last row of the table

is the value of each

is the value of each

general

the same

rule, the

transfer.

larger
TABLE 1.

Parameter

Range

a(Inclividual Measurement)

a(Mean)

Capaci tors

Al A2 A3 A4 Mean

(ppm) (ppm) (ppm) (ppm) (ppm)

0.022 0.080 0.064 0.036 0.046

0.009 0.020 0.020 0.012 0.015

0.003 0.007 0.007 0.004 0.005

a(ppm)

g(ppm/day)

Y(ppm/°C)

a(RES)(pprn)

TABLE 2.

DAILY VALUES FITTED TO a+ st + yA T

t = time in days

Al A2 A3 A4

-147.290 -270.11 5 -325.593 -1 72.455

-0.0012 -0.0030 -0.0027 -0.0021

3.2667 2.8826 2.6822 3.8676

0.006 0.004 0.006 0.004

AT = Tu.. - 30.00 °

TABLE 3.

CHANGES IN CAPACITANCE DUE TO SHIPMENT

Min. Change

Max. Change

Ave. Change

Change for 9 Months

Al

(ppm)

0.071

0.388

0.194

0.123

A2

(ppm)

0.047

0.162

0.104

0.426

A3

(ppm)

0.030

0.209

0.083

0.193

A4

(ppm)

0.020

0.111

0.061

0.081

Mean

(ppm)

0.009

0.180

0.069
In one shipment, capacitor A3 changed 1.24

was not included in Table 3, since in a normal

this change would be sufficient to cause this

excluded from all calculations.

Concl usi on

ppm. This value

MAP p rocedu re

,

capacitor to be

Calibrations using the transport standard appear to offer

significant improvement over the normal method of shipping

capacitors to NBS. At present, a 5-ppm uncertainty in value must

be assigned to capacitors shipped to NBS for calibration; a major

portion of this uncertainty is an allowance for changes due to

shipping. Using the transport standard, this uncertainty can be

reduced by approximately one order of magnitude.

There are several improvements that will be made on future

transport standards. For example, the enclosure is too heavy and

bulky for easy handling. Thus a reduction in both weight and

size will be attempted.

The capacitors used were not initially tested for frequency

response. Since they have varying degrees of frequency

dependence, their range of applicability is limited. The

conductances of these capacitors are higher than normal

indicating that greater care must be taken in the assembly of the

package on future versions.

The temperature coefficients and drift rates of the

capacitors were calculated for a period of time between

shipments. A further study will be carried out to see if these

coefficients remain unchanged over the time interval of two

measurement periods with shipment of the standards in between.

If the coefficients do remain constant, a further reduction in

the uncertainty of predicted values for the standards will be

justified.

Acknowl edgement s

The authors would like to thank Mr. Robert E. Long and Mr.

Charles Levy for their help in the construction and testing of

the transport standard.

(1)

(2)

References

Cutkosky, Robert D. Capacitance bridge-NBS type

Bur. Stand. Report No. 7103, 1961.

2. Nat.

Cutkosky, Robert D. Actual and passive direct reading ratio

sets for comparison of audio-frequency admittances. J. Res.

Nat. Bur. Stand., 68( 4) : 227 -236 ; 1964 October. Vol. 68C, No.

4, October 1964.
Appendi x Capacitors are exposed to a variety of mechanical shocks

during use in the laboratory or during shipment. Great care in

packing standard capacitors will, at most, reduce the intensity

of the shock incurred in shipment; in the laboratory, a capacitor

receives some degree of shock every time it is moved. Capacitors

of the type used in the transport box .have parallel plates fixed

in position by support rods. Mechanical shock tends to change

the geometry of the plates or the position of the plates relative

to the enclosure.

The mechanical stability of the standards was of utmost

importance since it was intended that they be shipped from six to

ten times annually. Therefore, six tests of stability were

devised to select those standards most suitable for shipment.

Since only a limited number of capacitors were available as

candidates for testing the transport box, absolute limits of

performance could not be established. Instead, those capacitors

of the available groups which appeared to be "best" were chosen.

The six tests used were designated as ORIENTATION, SMALL

ANGLE, TILT, SOFT KNOCK, HARD KNOCK and DROP TESTS. The test

protocols and the changes of capacitance measured are outlined

bel ow.

ORIENTATION: The capacitor is measured while in an upright

(vertical) position and while in the four posible horizontal

positions. The maximum difference with all measurements

considered is determined. Any change greater than 10 ppm is

usually evidence of a faulty capacitor. This test was used to

determine gross instability.

SMALL ANGLE: The capacitor is measured in the upright

position and on a 3° inclined plane. Four measurements are made

on the inclined plane with the capacitor being rotated 90°

(around the vertical axis) after each measurement. The maximum

difference with all measurements considered is determined. Since

most laboratory tables are not exactly horizontal, the offset

from horizontal would cause error if the capacitor were

excessively sensitive. Differences of several tenths of a ppm

were evident in some capacitors. Only those with a negligible

change were used in the box.

Aft er measurement of capacitance

,0

TILT: in the upright

position the capacitor is tilted 9

0*^ and returned to the upright

position for the second measurement. The test is repeated for

each side and the bottom of the capacitor, i.e., five tests in

all. The net difference C(UPRIGHT)^ - C(UPRIGHT)2 is determined.

Since changes with position should be completely reversible for a good capacitor, any measurable net change was cause for rejection.

SOFT KNOCK: The capacitor is measured before and after

being struck by the head of a pendulum. The head is covered with

8
soft rubber. The force generated by the pendulum is roughly

equivalent to that generated by a mild wrist action motion of the

human hand. The force is in a horizontal plane and it is applied

to the four sides and corners of the capacitor. The differences

C(BEFORE) - C(AFTER) are determined. A minimum change was the

criterion for selection of capacitors from those available as

candi dates . HARD KNOCK: The test procedure is the same as that for SOFT

KNOCK with the exception that the soft rubber head is replaced by

a hard plastic head. In the SOFT KNOCK test the force has a relatively long duration with a gaussi an-1 i ke shape; in the HARD

KNOCK the force tends to be impulsive with a more abrupt rise and

fall . DROP TEST: The capacitor is measured in the upright position. A

2" plate is inserted under one side of the capacitor raising that

side off the table. The plate is quickly removed allowing the

capacitor to drop back in its initial upright position and the

capacitance is measured again. Differences C(UPRIGHT)i

C(UPRIGHT)2 are determined for tests repeated for the four sides

of the capacitor. The applied impulse is in the vertical direction. Capacitors were chosen on the basis of minmum change in

this test.

The results of these six tests for a large number of 1000 pF

capacitors are shown in Figures 3-8. Many capacitors not

specifically considered for the transport standard are included

and, therefore, these graphs indicate the general performance of

the 1000 pF capacitors.

The graphs are histograms for the various tests. The Y axis

gives the percentage of capacitors which fall into specific

ranges of maximum change given by the X axis. The X axis has a limit of either 20 ppm (Figure 3) or 2 ppm (Figures 4-8). The

tests revealed a few capacitors with changes far beyond these

limits. In these cases the position of the mark on the graphs

relative to the limiting value indicate only that they are beyond

the limit and are not intended to show the actual change

measured .
15

CO

orO 10

o

<

O 5

ORIENTATION

PERCENT OF CAPACITORS

VS

MAXIMUM CAPACITANCE CHANGE

T

6 8 10 12 14 16

CAPACITANCE CHANGE (PPM)

FIG.3 Histogram of the fractional numbers of capacitors for which different values

of maximum capacity change were caused by the Orientation Test.

UJo

<

O

<

Q_

<

O

Ll_ O

20 -

15

-

10 -

SMALL ANGLE

PERCENT OF CAPACITORS

VS

MAXIMUM CHANGE IN CAPACITANCE

HI P0.2 0.4 0.6 0.8 1.0 1.2

CAPACITANCE CHANGE(PPM)

1.4

FIG.4 Histogram of the fractional numbers of capacitors for which different values

of maximum capacity change were caused by the Small Angle Test.

10
90 -

80 -

70

CO

o 60

1-< 50

Q_

<

O 40

Li_ o 30

20

10

TILT

PERCENT OF CAPACITORS

VS

MAXIMUM CAPACITANCE CHANGE

0.2 0.4 0.6 0.8 1.0 2.0

CAPACITANCE CHANGE (PPM)

FIG. 5 Histogram of the fractional numbers of capacitors for which different values

of capacity change were caused by the Tilt Test

50 -

o

<

CL<O

U-O

30 -

20 -

10 -

SOFT KNOCK

PERCENT OF CAPACITORS

VS

MAXIMUM CAPACITANCE CHANGE

XI.

1 \

1 1 I V~r0.2 0.4 0.6 0.8 1.0 1.2 2.0

CAPACITANCE CHANGE (PPM)

FIG. 6 Histogram of the fractional numbers of capacitors for which different values

of maximum capacity change were caused by the Soft Knock Test.

11
50 -

40 ^

CO

cro

o

^ 30

<

o

.o 20

10-

HARD KNOCK

PERCENT OF CAPACITORS

VS

MAXIMUM CHANGE IN CAPACITANCE

V^

0.2 0.4 1.2 1.4 1.6 2.0

CAPACITANCE CHANGE (PPM)

FIG. 7 Histogram of the fractional numbers of capacitors for which different values

of maximum capacity change were caused by the Hard Knock Test.

50 -I

U~) 40

O

I-

<

30-

o

?:5 20

10 -

DROP

PERCENT OF CAPACITORS

VS

MAXIMUM CHANGE IN CAPACITANCE

Vi

1 f=^

^

-=»-T-V

0.2 0.4 0.6 0.8 1.0 1.2 1.4 CAPACITANCE CHANGE (PPM)

1.8 2.0

FIG. 8 Histogram of the fractional numbers of capacitors for which different values

of maximum capacity were caused by the Drop Test.

12
NBS-n4A (REV. 2-8CI U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS IN 1162

2. Performing Organ. Report No 3. Publication Date October 1982

4. TITLE AND SUBTITLE

Transportable 1000 p"^ Standard for the NBS Capacitance Measurement

Assurance Program

5. AUTHOR(S)

George Free and Jerome Morrow

6. PERFORMING ORGANIZATION (if joint or other thon NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

W)^ Final

9, SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP) same as #6

10. SUPPLEMENTARY NOTES

[^J Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

A capacitance transport standard has been constructed for use in the National

Bureau of Standards Measurement Assurance Program. The transport standard was designee

so that variations in ambient temperature and mechanical shock would have a minimal

effect on the value of the internal reference capacitors. A significant improvement

in stability of 1000 pF capacitors during shipment and in the laboratory has been

achieved through this design.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calibration; measurement assurance; measurement assurance programs; reference

standards; standard capacitors; standard qualification; transfer standards.

13. AVAILABILITY

\_^ Unlimited

I I For Official Distribution, Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C

20402. Q Order From National Technical Information Service (NTIS). Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

15

15. Price

$2.75

Vr U.S. GOVERNMENT PRINTING OFTICE : 1982 O—360-997 (2271) USCOMM-DC 6043-P80

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Public Law 89-306 (79 Slat. 1127), and as implemented by Executive Order 11717(38 FR 12315, dated May II, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
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